X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. X-ray reflectometers typically operate by irradiating an area on the surface of a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a profile of interference fringes, referred to as an XRR spectrum. Properties of the sample layers can be estimated by analyzing the XRR spectrum. Several X-ray reflectometers have been described in the patent literature, such as in U.S. Pat. Nos. 6,512,814, 5,619,548 and 5,923,720, whose disclosures are incorporated herein by reference.
XRR is sometimes used for inspecting patterned wafers. For example, U.S. Pat. No. 6,754,305, whose disclosure is incorporated herein by reference, describes XRR methods for measuring the thickness of thin films on patterned semiconductor wafers in which the feature size is smaller than the measurement spot.
XRR may also be used for in situ inspection, i.e., during the deposition of thin film layers on a wafer. Such a system is described, for example, in U.S. Patent Application Publication 2001/0043668 A1, whose disclosure is incorporated herein by reference. A deposition furnace is provided with X-ray incidence and extraction windows in its side walls. The substrate upon which the thin film has been deposited is irradiated through the incidence window, and the X-rays reflected from the substrate are sensed through the X-ray extraction window.